帮我讲下题目要求的是什么？... （要求解什么）

Common Subsequence
Time Limit : 2000/1000ms (Java/Other) Memory Limit : 65536/32768K (Java/Other)
Total Submission(s) : 2 Accepted Submission(s) : 1
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Problem Description
A subsequence of a given sequence is the given sequence with some elements (possible none) left out. Given a sequence X = another sequence Z = is a subsequence of X if there exists a strictly increasing sequence of indices of X such that for all j = 1,2,...,k, xij = zj. For example, Z = is a subsequence of X = with index sequence <1, 2, 4, 6>. Given two sequences X and Y the problem is to find the length of the maximum-length common subsequence of X and Y.
The program input is from a text file. Each data set in the file contains two strings representing the given sequences. The sequences are separated by any number of white spaces. The input data are correct. For each set of data the program prints on the standard output the length of the maximum-length common subsequence from the beginning of a separate line.
Sample Input
abcfbc abfcab
programming contest
abcd mnp
Sample Output
4
2
0

样例输入两个字符串 str1 和 str2
输出 str1 和 str2 的最长公共子序列的长度
e.g:
str1="a b c f b c";
↑↑ ↑ ↑
↓↓ ↓ ↓
str2="a b f c a b";
str1 和 str2 的最长公共子序列是: 'a','b','c','b'
so输出的是 4
注意：子序列不是子串，子序列是这些字符不是连续的 但是有先后顺序；子串是连续的。

序列的子序列是将所给序列略去几个元素或者一个不略。给出一个序列X = ，另一个序列 Z = 是X的子序列 如果存在一个序列X的指数的严格递增的子序列 即对于所有的j = 1,2,...,k, xij = zj。例如，Z = 是含指数序列<1, 2, 4, 6>的 X = 的子序列。现给出两个序列X和Y，要求找出X和Y最大长度公共子集。 输入数据来自文本文件。文件中的每个数据都由两个字符串组成来表明所给序列。各个序列由空格隔开。输入数据是正确的