Where is your log data coming from? A good reference may be from the vendor providing the log data.
Is your log data in Elastic Common Schema (ECS)? Here is a link to ECS docs that define some of the fields you are asking about ECS Field Reference | Elastic Common Schema (ECS) Reference [8.11] | Elastic
agent: Agent Fields | Elastic Common Schema (ECS) Reference [8.11] | Elastic
host: Host Fields | Elastic Common Schema (ECS) Reference [8.11] | Elastic
As for @timestamp, checkout out What is the difference between @timestamp and timestamp?
@ fields are usually ones generated by Logstash as metadata ones, @timestamp being the value that the event was processed by Logstash.
Set @timestamp as the Time Filter field name when setting up your Kibana index pattern. This will allow Kibana's timepicker to filter on @timestamp field.