The proper way to catch a fly ball, which is taught from the very early leagues up through any instructional league, is to hold your glove up above your head, palm up, so your eyes have a perfect line of sight and you can "look the ball into your glove" (that is to say, watch the ball as it descends into your glove). It's proper form and the safest way to make a catch.
Willie Mays, whom "Wille Mays" Hayes gets his nickname from, was known to make riskier plays for the amusement of the fans. He was one of the first to adopt a "basket catch"; a type of catch where you hold your glove like a basket, below your waist. This required a bit more skill and ability to make some calculations quickly, as you can't line up the catch with your eyes and instead you have to rely on more of your judgement.
Lou Brown needs wins, and so wants the team to play using fundamentals and basic baseball procedures. There's no room for flashy plays or the errors that sometimes come with them. So he compliments Hayes on the catch, and then makes sure that he won't take risks in the future.