And a One and a Two
That's how we generally start tunes. And it's a very good way. It sets up the tempo and gets everyone on board for the "down beat. It can also highlight the basic rhythm: In this case 1 &a 2, which suggests an up-tempo swing.
It's called counting, and it's a very good tool. And it's powerful enough to get the entire band started. But a lot of people don't bother with counting when it comes to the actual playing. Counting can be just as important once the tune is underway.
Understand Internal Structure, the Subdivisions
The expression '1 &-uh 2' gives us an indication of the rhythm, in this case, medium to up-tempo swing in 4/4 time. Counting also helps us to understand the subdivision of the beats. Every type of note and tuplet has a corresponding count, and keeping track of these can help us interpret the rhythm more accurately.
Identify a Tune's Structure
The simple process of counting bars (1234, 2234, 3234, etc.) can unlock the structure of any tune. And if it's a particularly complicated arrangement, just write it down.
Polyrhythms & Other Tricky Stuff
It would be nice if we all could just hear complex rhythms and then play them. Most of us have to find some way of counting them. For example, I learned to play quarter note triplets with 'Pass the gol-durn butter'. I use 'serendipity' to count 5-lets. Doesn't really matter how you count things, as long as it works for you.
Embed Odd Groupings, Time Signatures
The easiest way to learn to play an odd time signature is to play it while counting. Try playing 7/8 for the first time without counting. It can't be done. Eventually you won't need the counting, but in the beginning, it's the only way to get there.
Fake It Till You Make It
Some things come easily, perhaps naturally. Other things may need a bit of help. There's no shame in counting. In fact, rigorous counting can take you places you wouldn't get to otherwise.
For best results, count out loud!