it means that the lecture is given by someone who is not the usual lecturer, usually a visitor invited (by the professor) to speak
Definitions of lectures on the Web:
en.wikipedia.org/wiki/Lectures
If you mean grammatically, no, the proper speech would be "the lectures have not (or haven't) begun yet".
There is no standard collective noun for the noun lectures, in which case a noun suitable for the situation is used; for example, a series of lectures, a program of lectures, a yawn of lectures, etc.
you can download lectures from internet
Fashionable Lectures was created in 1872.
Lectures on Faith was created in 1835.
Gifford Lectures was created in 1898.
Guildhall Lectures was created in 1959.
Tarner Lectures was created in 1916.
Warburton Lectures was created in 1768.
Bampton Lectures was created in 1780.
Hulsean Lectures was created in 1820.
Donnellan Lectures was created in 1794.