What's it called in chess when an attacked piece can't move because it will cause a less valuable piece to be captured?
I've a trouble with this terminology because in my language, we use the same word for both a pin and a skewer. So I still don't remember which of them is which. Btw, if the value is the same (e.g. two rooks), is it a pin or a skewer?
In a pin, the pinned piece blocks the access to the more valuable piece. If it protects the king, it is an absolute pin, else it is a relative pin.
If the directly attacked piece is of higher value than the piece behind, it is a skewer. I think this usually applies to same-valued pieces as well, like two rooks as per the example by mkubecek.
Sometimes a pin may cannot move because it needs to protect an important square behind it, like the back rank. I have seen those cases called (relative) pins as well. Technically this may be correct.
So pin and skewer have different effects:
A pin immobilises the directly attacked piece, as moving it would give access to something of more value.
A skewer makes the directly attacked piece move, so that you get access to the other one.
If the directly attacked piece is of higher value than the piece behind, it is a skewer. I think this usually applies to same-valued pieces as well, like two rooks as per the example by mkubecek.
Sometimes a pin may cannot move because it needs to protect an important square behind it, like the back rank. I have seen those cases called (relative) pins as well. Technically this may be correct.
So pin and skewer have different effects:
A pin immobilises the directly attacked piece, as moving it would give access to something of more value.
A skewer makes the directly attacked piece move, so that you get access to the other one.
For an evaluation of the effect, which of the two pieces is more valuable is only one of the criteria to consider. You'll need to take into account e.g. which of the pieces is protected, what is the value of the attacking piece, if there are other attackers and their values, which of the pieces can move with a check or a threat etc. For example, if a bishop pins/skews a rook and a queen, it's a problem whether they are protected or not and which one is in front. If a queen pins a bishop against a protected rook, it's often more of a problem for the queen than for the rook...
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