There are several icons which appear on claims to indicate whenever changes occur which you haven't seen yet:
blue dot: appears on new claims in a discussion which you haven't clicked yet.
speech bubble: clicking the speech bubble icon opens the Comments & History section of a claim. If it is blue, there are new unseen comments.
pencil: appears on claims which have unseen edits.
recycling bin: claims which appear in grey with this icon have been deleted.
arrow from bin: indicates a previously deleted claim which has been undeleted.
arrow from square: the claim has been extracted from another discussion.
upwards arrow: the claim was moved away from this location.
downwards arrow: the claim was moved to this location.
linked chain: the claim was linked in this location.
dot pointed list: two or more unseen changes have happened.
If a claim is “stacked” (i.e. you can see another card underneath the claim), there are children claims located below it. If the card underneath the claim is highlighted in blue, there are unseen changes, claims or comments located in the children claims
Additionally, claims can appear highlighted either grey or yellow. Grey claims have been deleted or moved away and no longer appear to users in the current location. Claims highlighted yellow have been marked for review by an Editor or Admin, indicating there might be an issue which needs further discussion.