❔ What is the use of datatype after function name?

I have a function signature that looks like this:

        public static IServiceCollection AddMongoRepository<T>(this IServiceCollection services, string collectionName) where T : IEntity

        public static IServiceCollection AddMongoRepository<T>(this IServiceCollection services, string collectionName) where T : IEntity

        public static IServiceCollection AddMongoRepository<T>(this IServiceCollection services, string collectionName) where T : IEntity

        public static IServiceCollection AddMongoRepository<T>(this IServiceCollection services, string collectionName) where T : IEntity

What is the use of <T> after the function name?