Modder alebal makes the missions using the powerful Mission Maker tool, and judging by the trailer above no two will be the same. You'll be transporting goods in trucks, flying helicopters, shooting up outposts, skydiving onto boats, and sniping from rooftops.
What's perhaps most impressive is that alebal isn't done: he adds 10 new mods with every major update, and the most recent batch were added last weekend. By the time you've got through all 70, there may well be another 10 to jump into.
Read the installation instructions at the bottom of the mod page—you'll need to download Mission Maker to make the missions work, by the sounds of it. If you're having trouble with a particular mission then it's worth viewing the comments on the mod page, where alebal suggests fixes. Some missions seem to require the popular Open All Interiors mod, and mission 44 requires this specific mod.
If I were you, I'd start with the 10 most recent missions if you can, because they'll be built with the latest version of Mission Maker.
If you're looking for other great GTA 5 mods, read Chris and Joe's piece here. And if you want to get over your GTA Online skepticism, Joe's excellent mini-series of GTA 5 roleplays is a good starting point.