A soft search is a check of a credit file that does not leave a mark other lenders can see. It is used for eligibility checks and quotes, and is visible only to the person whose file was searched, so it does not affect scoring.
In one line: A soft search checks a credit file without leaving a visible footprint, so it has no effect on a credit score.
How a soft search works
A soft search reads the file to assess likely eligibility, for example through an eligibility checker that estimates the chance of acceptance before any formal application is made.
Comparing five loans through eligibility checkers can record five soft searches, none of which other lenders can see. The same person can run as many as needed without the footprint that affects scoring.
Because only the file holder sees soft searches, they are also used for identity checks and account reviews without signalling new borrowing to other lenders.
Soft search vs hard search
A soft search is invisible to other lenders and does not influence a score. A hard search is recorded for others to see and can affect scoring, especially several in a short period.
Eligibility checkers use soft searches, while submitting a full application usually triggers a hard search, so the two often happen at different stages of the same process.
Primary source: Information Commissioner's Office: credit reference files