A filter is a device that traps large materials but allows smaller materials to pass through. For example, you pour hot water into a coffee filter, and as the hot water absorbs into the coffee grounds, the liquid passes through, but the chunky grounds don't. Now you get to drink the coffee, with no icky bits in it!
This process is called filtering, or filtration: it's the process of letting some things in (or out) while blocking or altering other things. Much like you can filter water or air, you can also filter images, videos, and even your thoughts and your speech, letting some parts of them go in or shine through, but making other parts of them stay out, or change on their way. In other words, to filter things is to control how much of them get to go in or come out.
You might filter your images so you look more glamorous, or filter your speech so you don't say any of the rude things you're thinking!
So, to filter or filtrate something is to control what goes in and what stays out. And to infiltrate a place or a group is to secretly go into it. Like an illness that infiltrates the human body, or like a virus that infiltrates a computer, infiltrating is a sneaky process that bypasses any protections or defenses in place. The infiltrators somehow squeeze their way in.
You can say this word either "IN fill trate" or "in FILL trate." Both ways are correct.
If you infiltrate a secret club, you sneak your way into it, even though they tried to keep you out. If you infiltrate a private meeting, you sneak your way inside it, even though you were supposed to be blocked from entering it. And if you infiltrate a group of bad guys, you fool them into thinking you belong to their group.