easy to; likely to; prone to; a tendency to~
Attached to a verb stem, やすい expresses that something is easy to do or that a person/thing tends to do something.
With action verbs, やすい means 'easy to do.' For example, この本は読みやすい means 'This book is easy to read.' It describes the quality of the object or situation that makes the action easy.
With verbs describing emotions or physical states, やすい can mean 'likely to' or 'tend to.' For example, 忘れやすい means 'prone to forgetting.' In this use, it often describes a habit or tendency of a person or thing.
The form becomes an adjective, so it conjugates like an い-adjective: past やすかった, negative やすくない, etc.