My husband is possibly getting a large bonus this year, maybe beginning of next (so this may be for 2018 taxes) but what is the tax implication of receiving a sum of money all at once?

The 2018 Federal tax law is likely to change, so keep watching the news. And remember state and local taxes as well.

The tax implication of getting a bonus is pretty much exactly the same as getting a raise; your income taxes will go up based on your marginal income tax rate.  Your taxes may go up more than you expect for various reasons: you move into a higher tax bracket, (that's pretty easy to understand), or your higher income or results in some of the numerous phase-outs, phase-ins, surtaxes and other "gotcha's" that litter the US Income Tax Code, (e.g., the so-called Pease limit on itemized deductions).

Bonuses are simply "compensation" and about the only thing that makes a bonus different than a regular paycheck is that the withholding is typically calculated at a flat "supplemental rate" of 25%.  Most states tax bonuses in a similar fashion.  Without knowing your personal details I can't say if the supplemental rate will completely cover your additional tax liability or result in a refund or a large amount owed. 

Tom Young