What does zero crossing period of a Auto correlation signify?
For a signal \$x(t)\$ of finite duration (say nonzero only for \$t \in [0,T]\$), the (unnormalized) autocorrelation function is
$$R_x(\tau) = \int_0^T x(t)x(t-\tau)\,\mathrm dt, ~\tau \geq 0$$
and of course \$R_x(\tau) = R_x(-\tau)\$ for \$\tau &lt; 0\$. Since \$x(t-\tau)\$
is nonzero only when \$\tau \in [\tau, \tau+T]\$, the lower limit on the integral
can be increased to \$\tau\$. Note that \$R_x(\tau) = 0\$
for \$|\tau| \geq T\$.  If \$t_1 &lt; T\$ is
the smallest positive real number such that \$R_x(t_1) = 0\$, then this means that
the signals \$x(t)\$ and \$x(t-t_1)\$ are orthogonal over the interval \$[\tau,T]\$,
(or over \$[0,T]\$ if you like).

If \$x(t)\$ consists of \$n \geq 1\$ periods of a single-frequency sinusoid, that
is, \$x(t) = \cos(2\pi nt/T + \theta)\$, then 
\$R_x(\tau) = \frac{1}{2}(T-|\tau|)\cos(2\pi n\tau/T)\$ for \$0 \leq \tau \leq T\$ and so
the zero-crossings are at times \$t_i = \frac{i}{T}, 1 \leq i \leq n\$. If\$x(t)\$
also contains signals other than the single-frequency sinusoid mentioned, there
can be other zero-crossings too.