Long ago, I was playing a tournament game against an opponent who was not a native English speaker. At one point midgame with the score still close, he tried RETIN(G)LE*, which I challenged off right away; I was able to play my own bingo in the same spot and went on to win comfortably. (The real word in those letters, GREENLIT, didn't fit on the board.)
I don't bring this up to mock non-native English speakers, of course. In fact, I have even greater admiration for a player who achieves sufficient word knowledge to play in top divisions without having a base of native language to start with. Such a player has to study harder to overcome that obstacle.
I'm sure most of you who are native English speakers would have challenged off RETINGLE* pretty quickly, too. We know what "tingle" means, and therefore we know that the RE- prefix almost surely doesn't work with "tingle." We may not be able to give a linguistic analysis of why it sounds wrong, but we know it does. But if you don't speak the language and all you know is that "tingle" is a verb and lots of verbs take RE- in front, RETINGLE* isn't an unreasonable guess.
You hear it said sometimes that definitions don't matter in Scrabble. That's correct - the rules of the game do not require you to be able to define the words you play, and for the most part trying to memorize definitions isn't a good use of your limited study time. But there are times when we all rely on our working knowledge of the language to help us out in a game, and knowing what words mean is part of that.
The most useful aspect of knowing what a word means, for Scrabble purposes, is knowing what part of speech it is. If you know PAIK means "to beat or strike," then you know it's a verb and you also know that PAIKED and PAIKING are good. (Not all verbs have regular inflecting forms like this, but most do; you'll just have to learn the exceptions, sorry.) If you know DORMIE means "ahead in golf by as many holes as remain to be played," then you know it almost surely doesn't take an S, and you also know DORMIER* and DORMIEST* are invalid, since there's about no plausible way a word with that meaning could have comparing forms. There are, of course, many tricky exceptions along the way: for example, in common parlance, the word ELFIN is an adjective meaning the same as ELFLIKE, so you'd figure ELFINS is no good, but it is - at least one of the source dictionaries for Scrabble also lists a noun sense for it. English can be a wonderfully ragged mess, and that's part of why it's such a good language for word gaming.
And even beyond knowing the part of speech, knowing what a word means often guarantees you'll be 100 percent sure of it. Knowing definitions never hurts, and now and then it can help. But if the plan isn't to sit around memorizing definitions, what's the way to get some benefit from knowing how certain words operate and what they mean?
When I started learning Scrabble words, the available learning programs did not include the definitions of words. But then Zyzzyva came along, and Zyzzyva has definitions. This was actually a big step forward. I wasn't trying to memorize definitions, but I found that just eyeballing them for a split second when they came on the screen in Zyzzyva got a surprising lot of them into my memory, especially with words I got a lot of repetitions on. The osmosis method, you might say. Since knowing parts of speech is the biggest benefit, sometimes I'd take a second to say to myself when a word came up, that's a verb, or that's an adjective and compares, or that's an interjection that doesn't take S, that kind of thing. Anything that gives your brain more information about a word with minimal added time and effort is good.
So if you can use a study program that provides definitions, I advise you to do so. Most desktop applications for learning words now have definitions, though some mobile apps don't. The only time I would suggest flat-out studying definitions would be learning which three- and four-letter words are not just regular nouns taking S. Even then, you probably don't want to make that too high a priority; for one, studying longer words will often give you -ED and -ING forms that fill in those gaps about as well. (And rote memorization is boring, so you don't want to make yourself do more of that than you really need. You'll study more and learn more if you bore yourself less.)