The Year of Jubilee
Every 50 years, the Jews were required to return any land they had bought back to its original owners, as well as set all their slaves (who wanted to be freed) free. This is what Levicitus 25:10 is talking about. Hebrew slaves were already ordered to be freed every seventh year (Ex 21:2), for slavery among Hebrew brethren was always meant to be temporary (as a means of settling debts). It was more like indentured servitude. The Jubilee emancipation is not described as being limited to just Israelites, but any and all slaves.
There are Christian groups who read the commands in the Old Testament and deduce which ones apply to us today. They might say (not being one of them, I don't know how they judge in this particular case) that we should observe the same rule if we lived in a society where slavery was permitted. Paul as much as said that slavery is to be avoided (1 Co 7:21), so it's not as though this provision in the OT is prevailing justification for allowing slavery.
I certainly don't read it as demanding action on our part to free other people's slaves. It's about setting your own slaves free.