The phrase "temp-to-perm job" is an abbreviation for "temporary-to-permanent job." The phrase refers to an employment position that starts as a temporary, short-term position (typically with a set time limit), but goes on to become a long-term, permanent one.
Temporary positions are often filled through "temp agencies," and are ideal for employers with a short-term need for a worker. This often occurs when a regular employee is on extended leave. Temporary positions also allow employers to test new employees to determine their capabilities and whether they are a good fit for the company. Very often, as temporary employees prove their worth, they're converted into permanent employees.