Characters advance through 20 levels. Each level grants more hit points and often a new class feature; certain levels (4, 8, 12, 16, and 19 for most classes) also grant an Ability Score Improvement you can spend to raise stats or take a feat.
Your proficiency bonus rises with level — not class — and is added to attacks, saving throws, and skills you’re proficient in. It starts at +2 and climbs to +6 by level 17 (see the table). That steady growth is why higher-level characters hit and resist so much more reliably.
Advancement uses either XP (crossing point thresholds — 300 for level 2, up to 355,000 for level 20) or milestone leveling, where the DM levels the party at story beats. Either way, proficiency and features unlock at the same character levels.