When browsing through the x86 Assembly Language Reference Manual, I came across this definition of an immediate operand:
Operands can be immediate (that is, constant expressions that evaluate to an inline value), [...]
If I got it correctly, an expression returns a value. Does a constant expression then return a constant value? Does inline value mean that it will be substituted by some other value after having undergone the process of assembly?
Does a constant expression then return a constant value?
Yes. An immediate operand creates a constant value in an opcode. Like mov eax, 3 creates an opcode consisting of several bytes - according to x32/x64 Intel Manual.
B0+ rb ib MOV r8, imm8 OI Valid Valid Move imm8 to r8. B8+ rw iw MOV r16, imm16 OI Valid Valid Move imm16 to r16. B8+ rd id MOV r32, imm32 OI Valid Valid Move imm32 to r32. C6 /0 ib MOV r/m8, imm8 MI Valid Valid Move imm8 to r/m8. C7 /0 iw MOV r/m16, imm16 MI Valid Valid Move imm16 to r/m16. C7 /0 id MOV r/m32, imm32 MI Valid Valid Move imm32 to r/m32. For example: For 32-bit-mode the following byte combinations do represent the valid opcodes. This means for mov destination, 3 there are several possibilities:
b0 03 mov al, 3 66 b8 03 00 mov ax, 3 b8 03 00 00 00 mov eax, 3 c6 45 00 03 mov byte ptr [xxx], 3 66 c7 45 00 03 00 mov word ptr [xxx], 3 c7 45 00 03 00 00 00 mov dword ptr [xxx], 3 B0h and B8h represent a one-byte opcode, C6h and C7h represent a two-byte opcode consisting of an opcode byte and a Mod/RM-byte coding the addressing mode - in this case indirect addressing. 66h designates an Operand-Size Prefix indicating that the following instruction uses an 16-bit(out-of-address-mode) operand - an exception compared to the normal 32-bit mode operand. An 8-bit operand is encoded with another opcode. The remaining bytes are the encoding of a 8/16/32-bit value of 3.
Does inline value mean that it will be substituted by some other value after having undergone the process of assembly?
No. Assembl(y|ing) is the process of incorporating constant values into resulting opcodes(seqeunces of bytes). In a way, everything is inline in assembly.
It just means that the expression must be a value that can be calculated during assembly. Examples of constant expressions include:
5 5+5 5*5 MyConstant (where this constant is defined somewhere else) MyConstant*4 Values for all above can be determined by the assembler and substituted with the value of the expression.
In contrast, non-constant expressions have values that cannot be determined during assembly. Examples include:
ah (a register) ah*al (expression containing registers) [0x800] (a memory location) 
mov eax, 1+2 and mov eax, 1+1+1 and other variants of constant expressions returning 3 all assemble into simply mov eax, 3. I.e. they are expressions that are evaluated by the assembler.