AWS cloudwatch metrics

I would like to know more details and impact on aws cloud watch metrics- aws docs does have much details on metrics

what is the difference between below metrics ?
What is the impact on application or aws instance if below alerts triggers ?

1. Http Server Errors GreaterThan 0 (Count) in the last 5 minutes was activated
2. Requests GreaterThan 100 (Count) in the last 5 minutes
3. Http 404 GreaterThan 0 (Count) in the last 5 minutes' was activated
4. Requests GreaterThan 500 (Count) in the last 5 minutes' was activated

Cloudwatch checking these errors in logs ?

These metrics are related to your load balancer. Here is my explanation:

1. the web server behind the load balancer throws an HTTP error with code 5XX - indicating that your server cannot perform the request. This can be due to several reasons such as Internal Server Error, Not Implemented (e.g., the server expects POST but the client sends GET), Gateway Timeout (e.g., the server executes a slow db query and the result does not come back in time), etc.
2. the number of requests completed or connections made is more than 100 - indicating exactly what is says.
3. the number of "Not Found" messages received by the clients - indicating that the client is requesting a page which does not exist in your application (for instance, https://stackoverflow.com/test)
4. the number of requests completed or connections made is more than 500 - the same as number 2 but indicating even more requests.

If all these alarms are triggered at once, there is probably high load on your server and it is not functioning optimally. More than that, though, is hard to say. You need to check the maximum number of errors. The most important one is 5XX (number 1). The load balancers, publish these metrics to CloudWatch and these don't have anything to do with your application log (if I understood the question correctly).