Two kinds of plug,
The one these posts are talking about is a small container with a 4 /6 or ten small seperate spaces for young plants to grow, "normaly from seed or cuttings,
these are gown ready to be re-potted or planted into the likes of hanging baskets etc,

The growers and gardencentre's tend to do their seeds this way as it saves compost and space.

It's a good way to buy your plants as the growers have germinated these "plugs" using thier heat/compost and have got the seeds/cuttings etc to the first level of germination at their cost,

It's now up to you to do the next stage of growing these plugs on to become mature.

One small draw back "if you buy plugs or any plant by mail order you take the chance of the plants being damaged.

(2)
The secong plug has a chain on it and fits into the sink,
it stops the water going down the plug hole,

(I could'nt resists this one !! sorry !! im going for a lay down in a dark room now)

But dungy what about the electrical plug, good for a quick 'buzz' if you touch the pins with wet hands.

Sorry as Dungy says: it is a small plant in a small 'plug' of compost. There are different sized ones.

The tiny ones are the cheapest, about 100 in a tray, these have only a couple of true leaves and some roots, they have to be potted on before planting out
Then there are big ones called pot ready and depending on the supplier intermediate sizes too that are more robust than the tiny ones.......

And it means you have to have lots and lots of growing room inside to pot up all the tiny little plug plants to grow them on in before they're planted out for summer. Plug plants are usually summer bedding plants - in other words, just a display for summer, so plants like bedding Begonias, busy lizzies and the like.

Thanks. I figured that's what the reference was. I usually get my annuals started that way.