I'll copy and paste from IGN. The gist (for me at least) is that the DLC is going to be expensive without a season pass, plus the season pass goes up in price from March1.
Bethesda announced its first three add-ons coming to Fallout 4: Automatron, Wasteland Workshop, and Far Harbor.
The first DLC released will be Automatron. "The mysterious Mechanist has unleashed a horde of evil robots into the Commonwealth, including the devious Robobrain," reads the description. You will be hunting them down and creating your own robots, choosing from hundreds of mods. Automatron will be available in March for £8 / $10.
Wasteland Workshop DLC
Wasteland Workshop will allow you to create and set traps for the inhabitants of the Wasteland, "from raiders to Deathclaws." You can tame the creatures you capture, or even have them face off against each other. Wasteland Workshop is coming in April, and will cost £4 / $5.
Far Harbor is the largest of the three announcements, and will be coming out in May for £20 / $25. It features a new case from Valentine's Detective Agency, and will have "the largest landmass for an add-on" Bethesda has ever created. There will be new quests, settlements, creatures, and dungeons, as well as brand new higher-level equipment.
Far Harbor DLC
Bethesda also announced more than £45 worth of new Fallout content will be coming in 2016. However, the price of the Season Pass been increased to account for the expanded DLC plan. Starting from March 1, it will go up from £25 / $30 to £40 / $50. If you've already purchased the Season Pass by that time at the lower price, you will get the additional content at no extra cost.