means realise what.

During the period when the polyphonic writers were pouring out their
most glorious and living stuff, in the first lame, crude fugues the
medium was being prepared for the triumphs of Handel and Bach; and in
the same way, while Bach was writing the G minor and A minor fugues (I
am not speaking of vocal music) some smaller men were working at what
was destined to grow into the symphony, sonata and quartet. These terms
are used here in their present-day signification. In the seventeenth and
eighteenth centuries such words as symphony and overture, and suite and
sonata, were interchangeable; but that does not at all concern us here.
The symphony or sonata or quartet form is what these early groups of
movements led up to. That these groups of movements originated in the
theatre is quite probable; this is indicated by the mere fact that the
word "overture" was frequently used to describe them. When the fugue was
in its fullest maturity composers were turning overtures out in vast
quantities. Our own Arne tried his hand at them, and no one looking at
_his_ would dream that the sonata form was so nearly ripe at the time.
Emanuel and Johann Christian Bach wrote them, and from these two Haydn
got the hint which he turned to such splendid account. Abel, Stamitz
and Wagenseil wrote them, and achieved nothing in particular. These
groups consisted of three or four movements, and we need not linger long
over them. It goes without saying that all the movements were short;
they consisted either of simple tunes or of series of harmonic
progressions broken up into figures or patterns. Of real development and
climax there is none; of such things as well-defined, characteristic
first and second subjects there is little sign. The themes were of the
formal mathematical type developed during the fugal period--a type that
"worked" easily, and in a way effectively, in the fugue itself, but was
unnecessary and, indeed, tiresome when contrapuntal working was not the