Help. What is this error?

You likely have GPU problems with that machine. Those pink lines are NOT normal.

These machines are known for GPU failures and the way it failed makes it easy to tell. You got unlucky that it failed, but you also lucked out because anyone who understands the problem knows this is a bad GPU. However, you should verify this by hooking it up to an external display. If it doesn't work or reproduces this problem, the GPU is bad.

Based on what I am seeing, I am inclined to think that error is a GPU issue. There are very few good used boards on the market, but Apple is repairing the 2011 for free in a lot of countries. I'm not sure if Russia is covered, but I would be surprised if it wasn't.

If Russia isn't covered by the recall, I'd suggest pulling the data off of the machine while it works and consider yourself extremely lucky that it didn't die young like most 2011 machines. Depending on what OS you have, it may not be a big deal to put the drive in another Mac but it's always best to pull the data while the system is intact enough to do it. Anything ~10.8 or newer often requires a much newer Mac for the transfer since Apple started dropping older models at this point.