Does anyone understand what these mean? 

The only reference I can find is
https://cs.chromium.org/chromium/infra/go/src/infra/appengine/sheriff-o-matic/frontend/elements/som-alert-view/som-alert-view.js?q=%22Tree+closers%22&sq=package:chromium&l=889&dr=C

It feels to me that the Tree closers for https://sheriff-o-matic.appspot.com/chromium.perf aren't actually as important as the Consistent failures...

Found this as well: https://cs.chromium.org/chromium/infra/go/src/infra/monitoring/messages/alerts.go?type=cs&q=infra/monitoring/messages+Severity&g=0&l=91
"TreeCloser is an alert which closes the tree. Highest priority alert."

I'm wondering how perf tree can even be closed.?

Gatekeeper sets the "tree status" on other projects' *-status.appspot.com apps, e.g.
http://chromium-status.appspot.com/ and their CQ will not merge any CLs while that app says their tree is closed.

chromium.perf does have an entry in the gatekeeper config: https://cs.chromium.org/chromium/build/scripts/slave/gatekeeper.json?g=0&l=615

But I'm don't see a chromeperf-status.appspot.com app though (and I'm also not sure SoM is interpreting that config the same way gatekeeper would). 

So these may just be regular alerts for chrome.perf sheriff's purposes.