To "secure" something means to keep something safe by making sure that it doesn't move, fall, slip, etc.

You can "secure" something by tying it, strapping it in, locking it, and so on. For example:

Make sure to secure the straps on your harness.

After you're finished, replace the cover and secure the bottom screws.

You can also use "secure" to mean "protect" in a wider range of situations:

They feel that the country should be making more of an effort to secure its southern border.