What means "fallbacks" in LN invoices?
As can be read in BOLT 11 of the lightning protocol spec 


  the f field allows on-chain fallback; however, this may not make sense for tiny or time-sensitive payments


it is one of the fields encoded into a BOLT 11 invoice


  f (9): data_length variable, depending on version. Fallback on-chain address: for Bitcoin, this starts with a 5-bit version and contains a witness program or P2PKH or P2SH address.


In case a payment can't be routed it is possible to pay this invoice with an onchain bitcoin transaction instead of opening a new channel. This obviously can't be done for sub dust payments and even for tiny payments above the dust limit this might not be economically viable.