Movement is critical in this game. Many weapons have a blast radius, so a movement left or right, forward or back will make you an easy target. Jumping and dodging is therefore critical—for two reasons. Most players will target blast weapons at the ground; and the splash damage is what needs avoiding. Being in the air plainly keeps you further from the ground. A dodge or jump is also faster than ground movement; so gets you out of the way.
There is a downside to movement. If you jump the distance, and therefore where you land, is predictable; this makes you vulnerable—especially to hitscan weapons. If an opponent is trying to hit you with a hitscan weapons, continuous small random movements, strafing left and right is better.
When playing against multiple opponents, and as your experience improves, another player might be lining up a shot from the side. Merely moving left and right means you'll be effectively stationary from his perspective; therefore adding backwards and forward movement, and moving in circles is even better. This technique is caled circle strafing.