The 2018 annual gift tax exemption is $15,000 per person (up from $14,000 in 2017). This means that you can gift anyone $15,000 per year without paying gift taxes and without needing to file a gift tax return. 

That means that you could give your parents each $15,000 (so $30,000 total) in 2018 without any taxes and without having to file a gift tax return.

If you are married, both you and your spouse could each give each of your parents $15,000 (so a total of $60,000) per year without any taxes and without having to file a gift tax return.

If you or you and your spouse were to give your parents $100,000, it would be more than the exempt amount for the year.  That means you would need to file a gift tax return that year.

As of 2018, every person gets a lifetime exclusion from gift tax of $11.2 million. Therefore, even though you would have to file a gift tax return for the $100,000 gift, you would likely not need to pay taxes unless you had already gifted over $11.2 million during your lifetime.

If you and your spouse gifted the $100,000, you would need to file a gift tax return, but you would likely not need to pay taxes unless you had already gifted a combined $22.4 million during your lifetimes.