A weather targeting filter is simply a weather based rule that you create, to either switch your adverts on or off, or modify your bids up or down in each of your geo targets, based on the weather there. We’ve written a whole guide around creating your weather filters which we recommend reviewing to get a better understanding of how filters work:

How to Add a Weather Targeting Filter to Your AdWords, Facebook or Instagram Campaign

How to Create Multiple Weather Targeting Filters (and why you might want to do this)

A single filter can have one or more weather condition attached to it, e.g. ‘rainy and windy’, or ‘hot or sunny’. However, a single filter can only ever have one action attached to it, which occurs when the condition is met e.g. ‘increase bid by 20%’. A secondary or 'default action' is then selected for when conditions across all of your filers are not met, e.g. 'don't show ads' or 'show ads at default bid'.

For AdWords campaigns you can have up to 3 filters per campaign. For instance, you might want to increase bids by 20% when it's sunny, and decrease bids by 10% when it's rainy. To do this you would need to set up 2 separate filters, because the action attached to each rule is different. To create additional filters simple click the + button.

Example: 

[Filter 1] Sunny > increase bid by 20%. 

[Filter2] Rainy > decrease bid by 10%.
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If conditions across both filters are true, the first filter will always override the second filter. E.g. in the above example, if the weather is both sunny and rainy, the bid will be increased by 20%.

Further Reading: How do time frames work for weather targeting?