Like if you start out as a level 1 rogue, and then at level 5 you take a level in bard. Then you level up bard to level 3, making you a level 8 character, can you then go back to rogue and keep leveling that up?
In fact, in 3.5, you were supposed to: 3.5 penalized you if you had classes with too-different levels, so you were supposed to keep them within 1 of each other by alternating. These were extremely unpopular rules that almost no one used, and Paizo replaced them with the much-better Favored Class rules, but they did exist.
Do note that Pathfinder generally makes multiclassing a poor idea; you tend to lose far, far more than you gain by doing so. Paizo was very heavy-handed in its attack on what it perceived to be too much multiclassing in 3.5.