These are verbs of motion and have a special grammatical system.
Each verb of motion has a "multidirectional" form and a "unidirectional". form.
The very general ideas are:
Multidirectional verbs express an action with no set direction, they just express the action of movement.
Unidirectional verbs express movement in a certain direction.
плавать - Multidirectional.
Они плавают = They swim / are swimming (around and about, not in any particular direction
e.g. Они плавают в бассейне - They swim / are swimming around the pool.
Here there is not set direction, they are swimming around.
плыть - to swim (in one direction):
Они плывут к берегу - They are swimming towards the shore.
Here there is a set direction, so the unidirectional form is used.
бегать / бежать - to run (multidirectional/unidirectional)
Я бегаю по городу - I run around the city
No set direction, therefore multidirectional (бегать)
Я бегу к магазину - I am running towards / to the shop.
A set direction, therfore the unidirectional form (бежать)
Other verbs: (multidirectional / unidirectional)
to walk - ходить / идти
to travel by transport - ездить / ехать
to fly - летать / лететь
to carry - носить / нести
to lead - водить / вести
to transport - возить / везти
to crawl - лазить / лезть