One way to search CloudWatch logs is to click on Log groups in the AWS Console, select a log group and use ctrl + f or use the search field to search for a string. This is not very efficient, especially if you have a lot of log groups.
Logs Insights allows you to search for a string across all log groups. You can also use the query language to search for more complex queries.
This query will search for all log groups that contain the string time, e.g. if you want to find out if there were Lambdas that timed out.
fields @timestamp, @message
| filter @message like 'time'
| sort @timestamp desc
| limit 20