How can I Filter Error logs

How can I filter Error logs through the other logs

input {
beats {
port => 5044
}
}

filter{
grok{
#I have no idea about GROK filter
}
}

output {

elasticsearch {
hosts => "localhost:9200"
manage_template => false
index => "%{[@metadata][beat]}-%{[@metadata][version]}-%{+YYYY.MM.dd}"
}

}