Source: Swimming Lessons London

Bilateral is breathing both sides, left and right. This is done every 3 strokes (counting both arms) so your breathing alternates from side to side.

It is important to breathe bilaterally to keep your stroke  symmetrical.

It will enable you to swim alot faster.

Drills
One-arm Freestyle To practice breathing to your left, leave your right arm at your side. Perform One-arm freestyle. Whenever you need to breathe, roll your head to your weaker side. If you’re having trouble with this drill, grab a board. Grip the board with your right hand and lock your elbow. Perform One-arm freestyle with the board. As you roll your head to the left side, the board should help keep you afloat. This will make it easier to breathe.

Use a Single Paddle After you’ve mastered breathing during One-arm freestyle, take away the board. Place a paddle on your left hand and swim regular freestyle. As you swim, you will most-likely feel lopsided. Focus on evening out your stroke, especially when you breathe. Breathe every three strokes. Try to make breathing to both sides feel identical.

Read more at: https://www.swimoutlet.com/guides/how-why-to-bilateral-breathe

Read more: http://www.swimsmooth.com/bilateral.html