Lines 10-15 Summary

Get out the microscope, because we’re going through this poem line-by-line.

People are not going
To dream of baboons and periwinkles.

So the people in the houses, with their boring, beltless nightgowns, are going to have lame dreams.
No dreams about baboons (those funny monkeys with blue faces and the pink butts).
And no dreams about little flowers called periwinkles.
Why baboons? Why periwinkles? We're not sure. The two certainly don't have much in common. But hey, maybe that's the point—they're different, random, and just fanciful enough to get our creative juices flowing.
What we notice here is that our speaker seems to know what's gonna go down before it happens. He already knows what dreams may come.

Only, here and there, an old sailor,
Drunk and asleep in his boots,

It looks like the speaker has found a few crazy old sailors to liven up the scene. Thank goodness.
We'd usually expect sailors to be by the sea, but here they are scattered throughout some (presumably) suburban houses.
However, sailors probably have lived through some exciting experiences: storms, foreign lands, new cultures.
They've traveled, seen the world, and are definitely not your typical suburban house dwellers.
Old and drunk, the sailors are passed out in chairs, on beds and floors, and even in front yards still wearing their boots. They must be pooped, if they couldn't even bother taking their shoes off before catching some Z's.
"[H]ere and there" suggests that there are more than one sailor doing this at the same time.
It's like a flash mob of booted, sleeping sailors.

Catches tigers
In red weather.

The speaker gives an example of what an un-boring dream for a sailor might be.
The sailor catches tigers (this is probably before tigers were protected by the government).
And he catches them in red weather.
Have you ever heard of the expression "Red sky at night, sailor's delight; red sky in morning, sailors take warning"? It's a classic old sailing adage, and it means that red weather at sunset is a good sign.
So, because it's night, and these dudes are sleeping, the fact that they're catching tigers in red weather probably means that the crazy dreams are more delightful than frightening. We mean, who wouldn't want to catch tigers? We're betting that's pretty thrilling.
So our speaker seems to be saying that a vivid imagination is nothing to be afraid of, rather it's to be enjoyed like a circus full of clowns and lion tamers.