These signify that you need to take corrective action to eliminate potential issues. I say potential issues because there are two types of warnings. The red circle warnings are serious while the yellow triangle warnings are not.
We used to call these 'hard errors' and 'soft errors' and you can usually ignore the soft errors - in fact you may have a reason for them. The soft errors usually occur if you add an Assign node but don't use it to do anything - No assignments were made.
I've used the Assign node (and have seen others do the same, so don't judge me) as simple placeholders or as a connection point pass thru. If you added an Assign and meant to actually assign a value to a variable, this warning lets you know you may have an issue.
Hard errors, however, should be taken seriously. As shown above, you may have placed a node in your flow but didn't connect to it, or from it. This 'orphan' node could cause your flow to fail because it won't know where it should go next.
You may have missing values in your node (like a File name) that are required for it to function correctly.
You also might not have completed your Error Condition steps by setting it up to notify or log potential errors.
So, don't ignore the warning signs. They're included to let you know you've got issues.