Implied probability is the probability a market price reflects. A YES price of 0.62 implies the market thinks the outcome has a 62% chance of resolving YES.
Every Polymarket price between 0 and 1 is also a probability between 0% and 100%. YES at 0.62 means "the market thinks there is a 62% chance this resolves YES" and pays $1 if YES wins.
If you think the real probability is 70%, buying YES at 0.62 has positive expected value: you are paying $0.62 for something worth $0.70 in fair odds. The harder question is sourcing a fair probability — that is what models like 5051x's exist to answer.
On tracked markets we compare Polymarket's implied probability against the 5051x fair-value line, and surface markets where the divergence is large.