Complex sentence: Most metal lids are slightly domed to serve as a seal status indicator: the vacuum in a properly sealed mason jar pulls the lid down such that the dome is concave, but an improper or failed seal or microbial growth will cause the dome to pop upward.
Split into simpler sentences: Most metal lids used today are slightly domed to serve as a seal status indicator. An improper or failed seal or microbial growth will cause the dome to pop upward.