Pokémon Unite currently features 5v5, 4v4, and 3v3 team battles that all have the same basic gameplay mechanics.

The main object of the game is to KO wild and opponent Pokémon to collect energy, and then deposit that energy into an enemy goal. The amount of energy you can carry before depositing increases with level, starting with 30 at level 1 and increasing to 50 at level 15. Upon deposting your energy in a goal, the goal's Hit Points will depelete and you will be rewarded with experience. Once a goal's Hit Points are fully depleteted, the goal will be destroyed and the next goal will open.
At the beginning of a match, your Pokémon will be level 1 and at your lowest evolution (if applicable). As the game progresses, you will level up. Leveling up unlocks evolutions, new abilities, and increased stats.
Boss Pokémon:
Each map contains a main boss Pokémon, and Remoat Island, the 5v5 map, contains two additional boss Pokémon. Each boss Pokémon gives increased experience points, but some have additional effects. Drednaw will give your team experience and a shield, Rotom will march towards a goal and leave it defenseless if he managers to enter, and Zapdos will leave every enemy goal defenseless.