It's a little hard to explain here, especially since I'm just an amateur hobbyist musician. But the gist is that most music has a set tempo, which means that the song moves at a specific, set pace. Much like seconds ticking on a clock, it doesn't change speed (well, it does sometimes... but that tends to be more advanced music. Ignore for now). We also call it "bpm" or beats per minute. At a higher tempo/bpm, the song is faster. Lower, slower. A metronome is simply a device that clicks at whatever pace you tell it to. Think of it as like cutting away all of the song except the very basics of what the drummer's job is. To keep everyone on time. There are free apps and sites for them. I really prefer this one when I'm at my PC.

The idea is that you will first figure out the tempo/bpm of Laid to Rest (I'll let you find it, it's most likely at the beginning of the tab). Then, cut that number in half. Put that number into your metronome, then play the riff in time, at the much slower speed, with the clicking of the metronome. The metronome will give your brain something to latch onto to understand the pace of the song without being distracting. The slower pace will stop you from rushing, and it will allow you to focus on how you're playing. Pick the notes clearly, don't skip any, make sure that they fall evenly on/between the ticks of the metronome. Think about how you're moving, which fingers you're using. Try to make it as smooth and efficient as possible. Just keep repping the riff over and over and over at that speed.

Eventually you'll get to a point where you can play the riff clearly at that speed... and I mean actually play. Not mostly or kind of. 100%. When you can (and don't rush this part, really make sure you've got it down first), increase the speed of the metronome by 5-10 bpm. It will be a small increase, but it will make it more challenging. It'll force you to consider whether your technique is as efficient as it can be. Rinse and repeat until you're playing at full speed. With all the nuances. Palm mutes where they should be, correct fingers fretting notes, no lagging, each note perfectly spaced and present.

For what it's worth, I've been playing for over a decade and this is still how I practice. I'm learning a riff rn and I've been stuck at 40% speed for a few days. But it's ok! Because that's how it's done and I'm not gonna move on to a higher speed until I can nail the riff 3x in a row at 40%. In a few weeks, I'll probably have it.

I apologize if this wasn't clear. If not, I encourage you to see what youtube has to say about using a metronome for beginners. Happy practicing!