The present participle, together with a the verb to be, is used to create continuous tenses.
Although a form of the verb, the present participle cannot be used as the main verb of a sentence. Trying to use it that way results in a sentence fragment: Playing in the lake.
To function as a verb, the present participle must be used with a helping verb: The children are playing in the lake.
Continuous tenses, also called progressive tenses, are used to describe a continuing action. The present, past, and future continuous tenses are formed with the present, past, or future of the verb to be and the present participle, i.e., the form of the verb that ends in -ing:
I am running for my life.
We were sitting in the hotel lobby.
This time next week, we will be celebrating your birthday.
In the comments to a post I wrote on the uses of sit and set, a reader brought my attention to an odd usage current in Britain. He provided this example: “The boy was sat on a rock by the harbour when the ship docked.”
The meaning of the sentence calls for a continuous tense: “The boy was sitting on a rock by the harbour when the ship docked.” The action of sitting was going on at the time the ship docked.
A post at the Oxford Dictionaries blog indicates that, while the usage may be popular among many speakers of British English, it’s not considered standard usage:
I’ve noticed several instances of […] ‘She’s sat at the table eating breakfast’ or ‘we were stood at the bar waiting to be served’. Aarrgghh!!! This construction is still regarded as non-standard.–OxfordWords blog
“Was sat” for “was sitting” seems to be a dialect form that has crept into the British mainstream. It is to be hoped (OK, I hope) that it won’t catch on with U.S. speakers.
According to the OED blogger, the aberration is limited to the verbs sit and stand:
It is 2pm and I am sat in my parents’ living room, talking to one of the cats.
We were stood at the bar waiting to be served.
If the action is continuous and uncompleted, you need an -ing verb:
It is 2pm and I am sitting in my parents’ living room, talking to one of the cats.
We were standing at the bar waiting to be served.