In a two-way market priced -110 / -110, each side implies 52.38%. Together that is 104.76% — the extra 4.76% is the margin, usually called vig or hold.
To de-vig, divide each implied probability by the total. 52.38 / 104.76 = 50%. That gives the market's opinion with the margin removed, which is the number worth comparing your own view to.
The same game can carry 2% hold in one place and 6% in another. That difference compounds far faster than most people expect.
This is why comparing prices matters more than picking winners for most people: the price you take is the part you can actually control.