What is this called?

You mean the bottom half of the pic showing the diamond plate 'plate' on the floor? That's built with about every trailer out there. Go to any metal supplier and get a piece of diamond plate.

 

Go to any trailer shop and show them yours and see what they say. I couldn't tell you what yours is. They can.

 

It’s steel. 1/8” thick I believe. Had a new one put on my last trailer. They stood it up, stitch welded it, laid it down, drilled new holes and installed new screws. I then had them weld it all the way across. Better than new. Original plate was thinner, and only 2’. New one was 4’. Much better.

 

Lowes/Home Depot sell diamond plates in the hardware section. they won't be as large as what is in your picture, but you can buy a a couple of pieces cheaper than at a trailer shop, as they get thiers from a vendor, and they will mark it up.

you can also try just about any welding shop, metal shop too, like an HVAC supplier...

 

Pretty sure I seen aluminum diamond plate at Menards the other day. Larger than I’ve seen at Lowe’s….

 

Can probably get it cheaper from a local machine shop than Lowe's, Home Depot etc

 

Diamond plate floor plate.

 

tread plate call any metal supplier in your area.