Whats the meaning of "gt" and "lte" in the field called "monitor.timespan"?

I just see in "heartbeat" index a field called "monitor.timespan" which contains the following:

  "lt": "2021-06-11T09:58:59.034Z",
  "gte": "2021-06-11T09:58:54.034Z"

What's the meaning of "lt" and "gte"? I would like to know if some of this dates simbolize the exactly moment when heartbeat checks the status of the monitor.
Thanks in advance

"Less than" and "greater than or equal", so the timespan represents time between 2021-06-11 09:58:54.034 and 2021-06-11 09:59:59.034 (though not including the exact millisecond of the latter.)