Please explain the pun(s) used by Maria while having a conversation with Sir Toby about Sir Andrew. and by Sir Toby as he speaks to Sir Andrew.Act...

Maria and Sir Toby Belch of Shakespeare's
Twelfth Night, introduce the audience to the lower class characters
of Illyria, and as such their language is more bawdy that that of Orsino and Olivia. 
Also, Sir Toby Belch is meant to be a comical and festive character.  So, he has fun
with double entendres such as the word except when he declares ,
"Why let her except before excepted" (1.3.7) parodying the legal jargon commonly used in
leases and contracts:  exceptis exceptiendis ["with the exceptions
previously noted"].

He also employs the literary device
called ploce as he switches the sense of Maria's word,
confine meaning "to limit oneself," changing its meaning to
"clothe":

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Maria:  Ay, but you
must confine yourself withing the modest limits of
order.

Toby:  Confine?  I'll
confine myself no finer than I am.  These clothes are good enough to drink
in....(1.3.8-11)

Then when
Maria scolds Sir Toby for having brought in a foolish knight, Sir Andrew Aguecheek, Toby
objects, claiming that Sir Andrew is bold and handsome.  Sir Toby adds that Sir Andrew
is polygot and "hath all the good gifts of
nature"(1.3.28).  Maria, punning on the word
nature, says, "He hath indeed all, most
natural;..."(1.3.29),suggesting instead that Sir Andrew is a "natural" meaning as it did
in Shakespeare's time "a natural fool or an idiot."

When
Maria is greeted by Sir Andrew, who is truly a "natural" in Maria's meaning, he thinks
Sir Toby's urgings to accost her mean that her name is Accost:  "Good Mistress Mary
Accost."  Sir Toby tries to explain that his meaning is to "assail her," which implies
seduction.  Maria joins in the bawdy talk and tells Sir Andrew to bring his hand to th'
"butt'ry bar [a storeroom] and let it drink"(1.3.65). Crassly, Sir Andrew asks her
"What's your metaphor" (1.3.66), and Maria replies, "It's dry, sir"(1.3.67), suggesting
impotence on Sir Andrew's part.  For, in Shakespeare's time, a man with a dry hand was
believed to be impotent.  When Sir Andrew still does not catch on, Maria indicates that
she is out of jokes with another somewhat sexual word, "I am
barren."

Likewise, Sir Andrew does not understand Sir
Toby's sexual puns such as

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An thou let part so, Sir Andrew, would thou
mighst never draw sword [male reproductive organ] again.
(1.3.55)

and when commenting
on his hair's appearance, Sir Toby says,

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Excellent.  It hangs like flax on a distaff
[rod,male organ], and I hope to see a huswife take thee between her legs and spin it
off[syphllis].
(1.3.99-101)

Another pun that
Sir Andrew misses is in this line:

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Sir Andrew:  Faith I
can cut a caper[frisky leap, dance].

Sir
Toby:  And I can cut the mutton to't [type of salted berry eaten with
mutton]. (1.3.111-112)

These
exchanges among Maria and Sir Andrew and Sir Toby indicate the high-spirited nature of
Sir Toby Belch, especially.