Theory:

The Crocodile is a small poem, written by Lewis Carroll in his famous book, Alice in Wonderland. Lewis Caroll is an English writer who has written children fiction stories and plays - which includes the famous Alice in Wonderland.
Crocodile is a reptile that lives both on land and on water. It is commonly found in Africa, Australia, Asia and America. It is a carnivorous animal (that eats only meat). It is an aggressive animal that has a rough, scaly skinpowerful jawsshort legs with claws with which it hunts its prey. Crocodiles generally eat fishes, birdsfrogs etc.
 
crocodile-1.jpg
How doth the little crocodile
     Improve his shining tail
And pour the water of the Nile
     On every golden scale!
How cheerful he seems to grin,
     How neatly spreads his claws,
And welcomes little fishes in,
     With gently smiling jaws!
                                                             Lewis Carroll
 
crocodile.png
 
Crocodiles are seen in parts of Africa, as mentioned above. The famous river Nile is home to many beastly crocodiles in Africa. The poem mentions the river Nile, and hence could be set in the Nile background.
 
The poet describes a small crocodile. He wonders how does (doth was traditionally used for "does") the crocodile makes its tail look better by pouring water from the River Nile on its scales that makes it looks shining like gold. The poet says that the crocodile's scaly tail is shining, and each scale looks like a golden scale. The crocodile seems to smile with its wide jaws and looks very happy and excited. Why is it happy? Because it is about to catch a fish. The crocodile has neatly spread its claws (a sharp curved nail on each foot). Its jaws (teeth) looks like its smiling, but it is making a hunt for the fish. The fish does not know the trap set by the crocodile, it sees the shining tail, golden scales, smiling teeth only! The crocodile lets the fishes happily in its mouth and eats them.  
 
The poet tries to bring about the meaning that all that glitters are not gold. We should not judge by outside appearances. We should always be careful about our enemies. We should see the motive behind every action. The crocodile tricks the fishes by showing itself as friendly by smiling, spreading his claws neatly, showing off his shining body etc. But inside it has dangerous motives of eating up the fishes. Similarly, we should not get  into the traps set by others.
The rhyming words in the poem are:
  1. crocodile - the Nile
  2. tail - scale
  3. grin - in
  4. claws - jaws  
Meanings of difficult words:
 
Words
Meanings
dothan expression for "does" - in old English
clawscurved sharp nails on each foot 
jawsmouth, including the teeth
gentlymildly, softly
grina broad smile
cheerfulhappy, jolly
Reference:
 
State Council of Educational Research and Training (2019). Term-1 English Standard-6. The Crocodile - Lewis Carroll (pp. 99-100). Published by the Tamil Nadu Textbook and Educational Services Corporation.