UK finance jargon - in plain English
Two different ways lenders check your credit file, with very different effects on your score.
A soft search is a basic check of your credit file that leaves no visible mark. Lenders use them for eligibility checks, identity verification, or pre-approval quotes. Only you can see soft searches on your report. A hard search is a full credit check carried out when you formally apply for credit, a mortgage, or a phone contract. It leaves a mark visible to other lenders on your file for 12 months. Too many hard searches in a short period can make you look like you are desperately seeking credit, which lenders view as a risk.
You use a comparison site to check what credit cards you might be eligible for. That is a soft search and leaves no mark. You then formally apply for the card. That is a hard search and appears on your file. If you do this with five lenders in one week, each one sees the previous searches and may decline you as a result.
Hard searches typically stay visible on your credit file for 12 months. Their impact on your score fades over time. Always check whether an eligibility checker uses a soft search before proceeding.
People apply to multiple lenders at once hoping to improve their chances. Each application triggers a hard search, and the combined effect can make approval less likely, not more.